SOME TYPES OF FLOWERLESS PLANTS. 223 
which result in the division of the protoplasmic contents of a cell into 
two independent portions, each of which is at length surrounded by a 
complete cell wall of its own. In Fig. 193 the division of the protoplasm 
and formation of a partition of cellulose in a kind of pondscum are 
shown, but the nucleus and its changes are not represented. 
Another kind of reproduction, namely by conjugation, is found in 
Spirogyra. This process in its simplest 
form is found in such unicellular plants 
as the desmids, Fig. 194. Two cells 
(apparently precisely alike) come in con- 
tact, undergo a thinning-down or ab- 
sorptive process in the cell walls at the 
point of contact, and finally blend their 
protoplasmic cell-contents, as shown in 
the figure, to form a mass known as a 
spore, or more accurately a zygospore, 
from which a new individual soon 
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Fic. 193.— Process of Cell-Multi- 
plication in a Species of Pond- 
scum. (Considerably magnified.) 
A, portion of a filament partly 
separated at a and completely so Fie. 194. — Unicellular Fresh-Water 
at b; B, separation nearly com- Plants (desmids), forming Spores 
pleted, a new partition of cellu- by Conjugation. (Much magnified.) 
lose formed at a; C, another A, a single plant in its ordinary 
portion, more magnified, show- condition; B, empty cell wall of 
ing mucous covering d, general another individual; C, conjuga- 
cell wall c, and a delicate mem- tion of two individuals to form a 
brane a, which covers the cell- spore by union of their cell-con- 
contents 0. tents. 
develops. In Spirogyra each cell of the filament appears to be an 
individual and can conjugate like the one-celled desmids. It is not easy 
to watch the process, since the growth of the filaments goes on mainly 
by day, in sunlight, and the spore-formation takes place at night, when 
growth is less rapid. It is possible, however, to retard the occurrence of 
